Once the report has been created, there are a couple of visual details you can add to it to make it more appealing!
Using the plus icon above the column label allows you to add a Superheader.
Once that is in place, you can use the E icon in the Superheader section to edit it (set the name, background colour and so on), the minus icon to delete the Superheader, or the arrow icons to move the borders of the Superheader around to fit it to match the columns you need.
There is no way to add Superheaders to rows.
You can, however, add a Row Separator!
Simply use the plus icon to add a row, and select the Row Separator option in the Display dropdown. You will be able to edit the Separator's label, as well as its colours.
Once you have everything in place, use the Preview button to see how your report will look like, once published!
Don't forget to set the Report visibility setting!